1. Field of the Invention
This invention relates to a geared electric motor and, more particularly, to a small-sized electric motor provided with a reduction gear and used for driving electrical equipment of an automobile such as a wiper, a power window or the like.
2. Description of the Prior Art
Heretofore, there has been used a geared electric motor shown in FIG. 3 for example.
That is, a geared electric motor 100 shown in the figure is provided with a plain bearing 102 between the shaft and a motor yoke 101 at the position near to the right side end in the figure, with a journal bearing 104 made from oilless metal between the shaft and a gear casing 103 fitted to an open end of said motor yoke 101 in the center of FIG. 3 and furthermore is provided with a plain bearing 105 between the shaft and the gear casing 103 at the position near to the left side end in the figure.
The armature shaft 107 of an armature 106 housed in the motor yoke 101 is formed with square holes 107a and 107b in each end thereof along the longitudinal direction of the armature shaft 107, and said holes 107a and 107b house balls 108 and 109 rotatably in order to reduce frictional resistance. The motor yoke 101 is provided with a stopper plate 110 in the end portion on the right side in the figure, which is in contact with the ball 108 housed in the square hole 107a provided at one end of said armature shaft 107. The gear casing 103 is provided with a thrust screw 111 in the end portion on the left side in the figure, which is screwed or unscrewed in order to adjust the distance from the ball 109 housed in the square hole 107b provided to another end of said armature shaft 107.
Said armature shaft 107 is supported in a rotatable state by the plain bearing 102 attached to the motor yoke 101 on the right side thereof in the figure, and the ball 108 housed in the square hole 107a provided to the right side end of the armature shaft 107 in the figure is in contact with the stopper plate 110 attached to the motor yoke 101.
The journal bearing 104 installed to the gear casing 103 supports the near center portion of the armature shaft 107 in a rotatable state and the plain bearing 105 provided to the gear casing 103 also supports the armature shaft 107 in rotatable state on the left side in a the figure.
In this state, to supply a prescribed current to a commutator 113 installed on the armature shaft 107 through a brush 113 in the forward rotational direction or the reverse rotational direction, the armature 106 rotates in the forward or reverse direction according to an electromagnetic force produced between the armature 106 and a magnet 114 disposed on the inner periphery of the motor yoke 101. Thereby a worm wheel 115 rotates, which is engaged with a worm 107c provided on the left end of the armature shaft 107 of the armature 106 in the figure, and so an output shaft 116 fixed to said worm wheel 115 rotates in the forward or reverse direction.
The forward or reverse rotation of said output shaft 116 actuates a linkage of a wiper device or a regulator of a power window device (not shown), thereby a wiper blade of the wiper device or a window glass of the power window device is driven.
Hereupon, the thrust gap between the screw 111 and ball 109 is adjusted by screwing or unscrewing the thrust screw 111 against the ball 109 housed in the square hole 107b provided to the left end of the armature shaft 107 in the figure. The thrust load applied on the armature shaft 107 when the output shaft 116 rotates a wiper blade of the wiper device in the reverse direction is received by said thrust screw 111.
However, in the conventional geared electric motor 100, since the thrust screw 111 receives the thrust load applied on the armature shaft 107, it is necessary to adjust the thrust gap every time by screwing or unscrewing the thrust screw 111. Therefore, there is a problem in that it is difficult to keep the motor 100 in good rotational condition because the adjusting operation is complicated.
Additionally, said armature shaft 107 is provided with balls 108 and 109 in the square hole 107a and 107b made in both ends thereof, and housed in the motor yoke 101 and the gear casing 103 wherein said ball 108 is in contact with the stopper plate 110 disposed to the motor yoke 101 and the thrust screw 111 is screwed in the gear casing 103 against said ball 109 housed in the square hole 107b. Accordingly, the members for supporting the armature shaft 107 are large in number causing another problem since the motor 100 is expensive in its cost comparatively.
Concerning the thrust load, there has been a wiper motor in which thrust load is received by a radial ball bearing as disclosed in Japanese Utility Model Publication No. 61-3248/86. However, the outer ring of the radial ball bearing is fixed to the gear case of the wiper motor by caulking or deformation of the gear case, and it is necessary to apply a buffer in order to cope with measuring errors of the armature shaft of the motor in diameter, and the radial ball bearing in width or length. Consequently it is difficult to assemble the motor automatically.
Furthermore, in case of applying a specific flanged ball bearing having a diameter smaller than that of the commutator in order to facilitate automatic assembly of the motor, the assembly is easy but the cost of the motor increases on account of the specially-made bearing, and in the motor disclosed in Japanese Utility Model Publication No. 62-23244/87, it is difficult to assemble the motor automatically since it is necessary to fix the outer ring of said radial ball bearing to the frame by screws screwed from the outside of said frame of the motor.